It is well known that spark ignition engines are generally less efficient (about 15 to 25% less) than compression ignition or diesel engines. This is because compression ignition engines operate with an unthrottled air intake, with very lean air-fuel mixtures and at higher compression ratios. Spark ignition engines generally are not made to operate in this way because combustion with spark ignition fuels (i.e., fuels with low cetane numbers) cannot be sustained in very lean, homogeneous air-fuel mixtures. Nonhomogeneous air-fuel mixtures (otherwise known as stratified charge) have been used to operate with very lean overall mixtures but, in spite of many years of effort, only limited commercial success has been achieved with this approach. To achieve higher engine efficiency with low cetane fuels is an important objective since it could provide an alternative to the use of diesel engines in passenger cars and light vehicles and have major implications for the future in the use of fuels such as methanol and ethanol derived from coal and biomass because such fuels are low-cetane fuels.
Another approach to achieving unthrottled, lean-mixture operation in spark ignition engines is to use some form of electromagnetic radiation to stimulate combustion in the cylinders. In this way flames may be made to travel through very lean mixtures and consume the fuel in the way they do with conventional, near stoichiometric combustion. Radiation assisted combustion could also reduce exhaust emissions; inherently, lean mixture operation can reduce oxides of nitrogen and carbon monoxide and stimulation of combustion by means of radiation in the thermal quench layer on the walls of the combustion chamber could also reduce hydrocarbon emissions.
Radiation assisted combustion could also be important in compression ignition engines in helping to reduce particulates or soot and in facilitating the use of lower grade fuels. It might also have an application for nonautomotive power plants and in some forms of chemical processing. The principal application to be considered here, however, is the unthrottled lean mixture operation of spark ignition engines.
Although not necessarily aimed at unthrottled, lean mixture operation of spark ignition engines, several methods have previously been proposed of using radiation to promote combustion. The types of radiation that have been considered extend over the range of electromagnetic radiation. They include:
(a) Electric fields and microwaves
(b) Visible and near visible light including lasers
(c) Ultraviolet light
(d) Nuclear radiation.
Viewed in a practical light, some of these methods present substantial difficulties. The use of nuclear radiation would appear to involve substantial safety questions. The concept of electric fields has not so far been demonstrated to be beneficial in engines. Laser sparks appear to be of little value because of the impracticality of using a laser system in a vehicle; also, although laser sparks can ignite lean mixtures, the use of laser light to increase flame speeds would require excessively large amounts of electrical power. In general, light sources in the visible and near visible range can only stimulate combustion by heating the gases in the flame and it is indicated that stimulating combustion by thermal means requires too much power in relation to the additional engine power that could be gained by unthrottled lean mixture operation.
The microwave approach appears easiest to implement with current technology. However, there are inherent difficulties with the use of microwaves. It has been shown that the input impedance of microwaves in a flame loaded, combustion chamber resonance system is significantly affected by the shape and location of the flame in the combustion chamber cavity. Because of turbulence and random motions of a typical flame in an engine cylinder, it would be difficult to keep the microwaves continuously coupled to the flame. Another problem is that the tangential component of the electric field generated by the microwave resonance dies out near the electrically conducting walls of a combustion chamber. Since it appears to be this component that drives the flame, it would seem that the flame would quench itself before it could reach the walls, thereby increasing emissions and reducing engine efficiency. Another difficulty is that turbulence and other gas motions appear to inhibit the transfer of energy from the electrical field to the flame. Because of all these difficulties, microwaves may not be the most effective way to stimulate combustion in spark ignition engines.
Ultraviolet light can stimulate combustion in an entirely different way. This form of radiation is absorbed by the oxygen molecules in the air-fuel mixture which disassociate into oxygen atoms. Oxygen atoms are highly reactive and if present in sufficient quantity will initiate combustion in the mixture. The ability of ultraviolet light to initiate combustion is a well documented effect (see, for example, R. G. W. Norrish, "The Study of Combustion by Photochemical Methods," Tenth Symposium (international) on Combustion, pp. 1-18, The Combustion Institute, 1965 and A. E. Cerkanowicz, "Photochemical Initiation of Sustained Combustion in Unsensitized Gaseous Fuel-Oxygen Mixtures," Ph.D. Dissertation, Stevens Institute of Technology, June 1970).
One proposed extension of this concept, called "preconditioning", is to irradiate the air-fuel mixture with ultraviolet light prior to combustion. Initiation of combustion is then achieved by creating a sufficient concentration of oxygen atoms next to the window of the light source (or by using some other form of ignition). Stimulation of subsequent combustion is purportedly achieved by the oxygen atoms created in the path of the flame by the prior irradiation of the air-fuel mixture with ultraviolet light.
The preconditioning method assumes that the oxygen atoms will stay in existence long enough in the unburned mixture to stimulate the flame as it moves into the unburned mixture. Towards the end of the compression stroke in an internal combustion engine, the air-fuel mixture reaches temperatures of about 600 K and at these temperatures the reaction times of the oxygen atoms are very short, of the order of microseconds. It can be questioned, therefore, whether the oxygen atoms created by prior irradiation will remain in existence long enough to stimulate combustion in the flame.
Another drawback of the preconditioning method is that it requires ultraviolet radiation of relatively short wavelengths, roughly in the range from 160 to 200 nm. Presently, there appear to be no reasonably efficient sources of ultraviolet light in this range. Rare gas, electrical discharge lamps have been used experimentally, but only a very small part of the output of these lamps is in the required range. Also exotic materials such as sapphire and lithium fluoride are required for the windows since ordinary quartz or glass generally does not have enough transmissibility in this range of wavelengths.